The Electric Grid
The network over which electrical energy or power is distributed is referred to as the electric grid. Generally, electrical energy is delivered from power plants to end users in two stages. These two stages are bulk transmission and local distribution. Bulk transmission, or “high voltage electric transmission,” is the transfer of electrical energy from generating power plants to substations. The portion of the electric grid that is involved in bulk transmission is referred to as the transmission grid. Local distribution is the delivery of electrical energy or power from substations to end users. The portion of the electric grid that is involved in local distribution is referred to as the distribution grid.
Management of the power running through the electric grid is important both to efficient power delivery and to grid maintenance. Electrical energy is difficult and expensive to store and therefore grid management is typically focused on substantially continuously matching production with consumption. Reasons to manage the electric grid efficiently include the following: unused electrical production facilities represent a less efficient use of capital (little revenue is earned when not operating) and by “smoothing” demand to reduce peaks, less investment in operational reserve will be required, and existing facilities will operate more frequently. Most noticeable to the electricity user is that failure to respond to changes in load in time can result in grid instability and grid failure.
A common method of grid management is load management, which is the process of balancing the supply of electricity with the load by controlling the load rather than controlling the output at the power plant. Examples of load management techniques include triggering circuit breakers and using timers. Residential and commercial electricity use often varies drastically during the day, and demand response grid management techniques attempt to reduce this variability based on pricing signals intended to influence end user behavior. Some load management techniques include predictive techniques and involve modeling based on past load patterns, weather and other factors.
Conventional load management techniques have numerous flaws. These flaws include slow response time, as well as interference with customer experience. In some cases, the slowness is inherent. In some cases, the response time is limited by the age of the grid infrastructure. Predictive load management techniques may fail to compensate for some types of events.
Further complicating grid management is the trend toward distributed power generation. Power from a larger number of sources complicates the matching process. Additionally, some power generation, such as wind power, is intermittent.
There is a need for grid management methods and systems that are able to handle the complexities of distributed power generation including generation from intermittent sources, and that respond faster to irregular and unpredictable events in a way that is relatively transparent to the end users.
2. Electric Thermal Storage Heaters
Electric space heating accounts for a substantial minority of heating in commercial and residential living space. Within electric space heating, Electric Thermal Storage (ETS) heaters are currently a niche market, originally developed in Europe during World War II. ETS heaters had a period of regionally-specific popularity in the United States during the 1980's and early 1990's when utilities promoted them as a means to deal with anticipated nuclear generated electric energy at night.
Conventional ETS heaters share the following basic architecture. The ETS heater has a heat sink surrounded by an insulated housing. The heat sink is often made of some type of brick. The brick for example is a type of ceramic brick that can be heated to a high temperature. An example maximum temperature of this ceramic brick is 1200 degrees F. Some bricks, for example conductive bricks developed at Quebec Hydro, West Montreal, Quebec, Canada, are able to achieve a higher temperature.
The ETS heater may further include at least one duct through the heat sink and housing to allow for surrounding air to be circulated past and heated by the heat sink. The ETS heater includes electric heating elements for generating heat and one or more fans for circulating air through the ducts. A room thermostat, that is, a thermostat for measuring the temperature of the space to be heated, is responsible for either directly or indirectly controlling operation of the fan. In the direct case, the ETS heater includes the room thermostat, which controls operation of the fan. In the indirect case, the ETS heater receives a heat call signal from the thermostat and uses that signal to control the fan. The ETS heater typically includes a second thermostat for measuring the temperature of the heat sink. In addition, the ETS heater in some implementations receives signals from an outside temperature sensor which measures the temperature outside of the space to be heated. This temperature is typically the outdoor temperature.
ETS heaters come in two types: room units and furnaces. The distinguishing factor between the two types is that furnaces connect into central heating systems while the room units pump hot air directly into a room. The ETS furnaces further subdivide into whether they connect into air or water heating systems. Within these basic categories, there are also distinctions based on how much energy the heat sink can store and how much power the system can draw. The ETS heater is sometimes the sole heat source, sometimes primary, and sometimes supplemental. For example, ETS furnaces are sometimes used as supplements to heat pumps. In cases where room units provide the sole or primary heat for a building, the ETS heaters are often connected to a main controller by means of a system of low or high voltage wires.
Sometimes an ETS system also controls other sources of heat. For example, an ETS furnace may be connected to a heat pump, and control when the heat pump is actuated.
ETS heater units are often connected together into an ETS system: a central control unit receives signals external to the house, for example the outside temperature sensor signal and an “available/not available” signal, and relays these signals to the individual ETS heater units via some type of local communications network. The individual ETS heater units in the system typically receive input from separate room thermostats with each ETS heater unit responsible for heating a separate “zone” in the area to be heated. Accordingly, an ETS system can consist of a network of a single furnace, a furnace with one or more room units, or multiple room units.
Most existing schemes for controlling the operation of the heating elements in ETS heaters involve establishing at each moment in time a current desired maximum tCDMax and a current desired minimum tCDMin for the temperature of the heat sink. These temperatures may be a function of readings of outside temperature sensors, or they may be a function of both readings of outside temperature sensors and historical fan behavior (which is linked to heat flow out of the bricks). In addition, there is a mechanism for determining that electricity is available for the ETS heater—“available” or “not available.” In conventional ETS heaters, this “availability” mechanism does not take into account information from the particular ETS heater. Examples include a timer, or an “offpeak/onpeak” signal sent by power line carrier signals (PLC) over the existing power lines from the utility. When electricity is available to the ETS heater, the ETS heater is not charging, and the current temperature tC is less than the current desired minimum heat sink temperature, then the ETS heater will begin to charge. Likewise:
Electricity AvailableNot chargingChargingtC < tCDMinStart chargingContinue chargingTCDMin < tC < tCDMaxContinue not chargingContinue chargingtCDMax < tCContinue not chargingStop charging
When electricity is not available, the ETS heater does not charge. Conventional ETS heaters typically have an emergency override feature that is often manually implementable.
The above controls are implemented in conventional ETS heaters using a variety of methods including mechanical, electrical, and hybrid mechanical-electrical systems.
The conventional control mechanisms for ETS heaters often result in significant and prolonged surges in the use in the first few hours of a nightly off-peak “Electricity available” period, followed by minimal to low levels of charging in the middle of the night. This is far from ideal in terms of providing cheap electricity from the point of view of the wholesale purchaser. From a grid management perspective, this is not a good method of flattening the load curve.
In addition, conventional art is not capable of balancing the responsibility for guaranteeing a warm home while simultaneously taking advantage of close to optimal charging profiles. For example, although most ETS-heated homes in a small utility's service area may need only to heat for three hours in a particular day, the utility cannot send out an “available” signal limited to the cheapest three hours of that day because some of the homes may in fact need more energy than they can draw in those three hours.
The heuristic control mechanisms described above become even less optimal in regions with increasing levels of non-carbon generation, where there can be dramatic and variable changes in supply and price.
In conventional art, high penetration of ETS heaters on a circuit of an energy distribution system presents a challenge. There is no method that is both reasonably equitable and close to optimal for dealing with distribution-level constraints. Given that: 1) conventional residential ETS furnaces can charge at up to 45 kW; 2) conventional residential ETS furnaces will often charge at an average of 4-6 kW over a week; 3) and that average household electric loads are typically on the order of 1 kW, the issue of distribution constraints is important be addressed once penetration of ETS heating exceeds, for example, a couple percent.
Some efforts have been made in the conventional art to address the issues associated with distribution constraints—specifically, efforts to avoid the simultaneous activation of heater charging. For example, in many of the ETS heaters, there are either mechanical or electrical mechanisms for introducing some randomness in beginning to charge during a “charge available” cycle. In one implementation, thermal relays take variable amounts of time (up to 3-5 minutes) to turn heaters on, even though all the heater sites may be reacting to the same sensed parameter. In some conventional heaters, the heater operates on an internal 15 minute clock that occasionally resets. Thus, heaters of this type responding to the same signal will stagger over a 15 minute period. In addition, some heaters will charge at a more rapid rate in colder weather, taking advantage of inherently greater capacity on the lines when they are colder.
There remains a need for a decentralized energy management solution that enables buildings with ETS heat to obtain electricity on a more efficient basis (i.e., without burdening the grid unnecessarily) and that likewise enables the grid operator to cost-effectively manage the balance of supply and demand. Further, there remains a need for an energy management system that includes management of distributed power generation including power from intermittent sources.